1. Field of the Invention
This invention relates to the field of active noise control, and in particular to an electronic stethoscope usable in a noisy environment.
2. Description of Related Art
Use of a stethoscope for auscultation, for example to detect lung noises or monitor heart beats, is often impossible in ambulances, medivac helicopters, and other emergency medical environments due to detection of extraneous noise and vibrations by the sensing device of the stethoscope, and because the signal output by the sensing device must compete with additional airborne noise that penetrates past the earpiece of the stethoscope and into the user's ear. In order to solve the problem of auscultation in a noisy environment, the stethoscope must limit both the direct detection of extraneous sounds by the sensing device of the stethoscope and the effect of noise which penetrates past the earpiece of the stethoscope. No such system currently exists, although solutions have been achieved for specialized situations. Such solutions have generally involved substituting an electrical transducer such as a piezoelectric microphone element for the vibration detector of a conventional stethoscope, and then applying electronic signal processing techniques to the resulting electrical signal.
There are several advantages to having a sensor with electronic output. First, the electronic output is amenable to filtering in order to receive the frequency band of interest. Thus, noise outside the frequency band of interest can be removed. Second, heart and lung sounds often get garbled by reverberation in the rubber tubes of conventional stethoscopes, but the electronic signal produced by the electronic sensor is not susceptible to such reverberations. Third, a sensor that generates an electronic signal is advantageous in that the electronic signal can be amplified and filtered to compensate for hearing loss specific to an individual physician. Finally, the electronic signal generated by the sensor can be used in conjunction with adaptive noise control techniques to further reduce unwanted noise.
The earliest forms of electronic signal processing in this context involved adaptive noise cancellation techniques based on subtraction of reference signals related to specific noise sources. For example, a DSP implementing a least means squared (LMS) algorithm was successfully used to remove unwanted 60 Hz noise which interfered with the recording of electrocardiograms (ECGs). For this application, the primary input signal was the ECG, which was correlated with a secondary input reference signal taken from a nearby electrical power outlet in order to obtain the part of the primary signal uncorrelated with the 60 Hz source of electrical interference.
In another application of the LMS filtering technique, a fetal ECG device was developed which cancelled out maternal heartbeat signal from a fetal heartbeat monitor. For this application, the primary signal came from a stethoscope placed near the infant, and the secondary reference signal was obtained from a stethoscope near the mother's heart. After removing the part of the primary signal correlated with the reference signal, the infant's heartbeat could be heard much more clearly.
More recently, rather than relying only on passive attenuation of external noise, research has focused on active noise control techniques. Here, active noise attenuation refers to the reduction of noise due to interference with a controlled secondary source of sound or "antinoise". No matter how noise-free the speaker's output, external airborne noise which penetrates to the listener's eardrum will still be a problem. In the case of a stethoscope, for example, external airborne noise that penetrates to the listener's eardrum and masks the relevant stethoscope signal can significantly interfere with the listener's interpretation of the signal. Since the source of noise does not come from the speaker driving signal, it cannot be controlled by simply passively filtering the speaker driving signal, but rather must be actively controlled.
The active control of sound in antinoise headsets is currently being investigated by many researchers. This headset research is divided between work using analog devices and digital (DSP) devices. Analog headsets have been under development for some time (see U.S. Pat. No. 4,445,675, and also U.S. Pat. Nos. 4,494,075, 4,644,581 and 4,856,118) and currently more than ten companies, including The Bose Corp., have made such headsets commercially available. Basic research on the design of DSP anti-noise headset systems is currently on-going. While the analog systems are less expensive, their cancellation performance is limited.
Unlike analog systems, a digital antinoise system can adaptively redefine its operating parameters in order to seek out the optimal way to cancel a particular noise problem. However, in practice, most DSP algorithms only remove periodic noise in ANC headsets. Periodic noise is much easier to cancel than broadband random noise. In emergency medical environments where most of the noise is periodic, the conventional algorithms are satisfactory. Nevertheless, there are many situations in which random noise cancellation is required.
Even where the external noise source is periodic, active noise control techniques by themselves may be inadequate to completely eliminate the background noise. Problems include inadequate reference sources for attenuation of both electronic noise in the primary signal and acoustical noise near the ear of the stethoscope's user, a primary signal which is too weak in relation to the noise sources, and differences in sound between the electronically filtered speaker output and the sound to which the user is accustomed. The present invention seeks to provide complete solution to these and other problems by using a variety of electronic processing techniques and by combining these signal processing improvements with improvements in the hardware by which the electrical signals to be processed are obtained.